This invention relates to devices used to grip coiled rod for injection into wellbores during wellsite operations. Coiled rod is manipulated downhole typically with continuous feed injection units that include gripper pads for gripping the well strings. One early such design is shown in U.S. Pat. No. 3,559,905 of Palynchuk, issued Feb. 1, 1971, in which a continuous chain with gripping blocks carried by the chain is used to inject the well string into the well. More recently, such continuous chain gripper systems have been described in U.S. Pat. No. 5,553,668 of Council, et al, issued Sep. 10, 1996. As disclosed in Canadian patent application no. 2,351,648 published Feb. 21, 2002 the continuous feed injection units may be suspended from the travelling block of a drilling rig and coiled rod is fed through a guide from a carousel into the continuous feed injection unit.
Gripper pads used previously with coiled rod are of the chain type. These chain type gripper pads have a tendency to wear in a manner called crowning. The individual pads tend to break down at their ends with the result that the gripper pad surfaces take on a convex shape. When the two chains of the continuous feed injection units are out of synchronization, the convex surfaces of one chain fall between the convex surfaces of the opposed chain. Pressure from the pads on each other then tend to bend the coiled rod, rendering it unsuitable for use. One way to avoid gripper pad crowning is to use hardened steel for the gripper pads, but the use of hardened steel tends to scar the rod. Another way to avoid gripper pad crowning is to make the chains run synchronously, but again this is expensive.